I've bought 2 or 3 machines from their refurbished store over the years and always had good luck with them.
[link|http://www.shopping.hp.com/product/outlet_store/refurb_desktops/compaq_presario/4/storefronts/EX325AAR%2523ABA|SR1950NX] (it's out of stock at the moment). $430. [link|http://www.skarulis.com/?cat=17|It works with Ubuntu].
I haven't bought that particular model, but have had good luck with Compaq Athlon machines. Use the extra money for the monitor or more memory or ...
HTH a bit.
Cheers,
Scott.